if its onto only, then that means for every element T(x), there is an element x which corresponds to it. And in onto, it doesn't matter if there are lots of distinct element x correpsonding to T(x). Such can be visualized like this:
|dw:1336818258901:dw|
now for one-to-one, for every element T(x), there must only be one element x corresponding to that. like this:
|dw:1336818376666:dw|